This invention relates to a technique for preventing fuel from dripping out of a suction piston of a variable venturi carburetor during low load operation.
As is well known, various kinds of carburetors have been adopted for automobile engines. Among them a variable venturi carburetor has been generally employed and equipped on automobiles in a wide range from some sport-type models to normal models because of superior transient responsibility and a reduced height of the entire unit.
However, more than a few problems to be improved remain in such variable venturi carburetors.
One of these problems is the fact that fuel periodically drips during low load operation.
As shown in FIG. 1, in a variable venturi carburetor 1, an amount of suctioned air varies in proportion to the degree of opening of a throttle valve 2 and a suction piston 3 opens or closes a venturi portion 4 corresponding to the amount of suctioned air. But, when the suction piston 3 is located at a position near a main nozzle 5 in a low load state, fuel, having been suctioned from a float chamber 6 via a well 7 and gauged in the amount thereof under the cooperation of a metering needle 8 integrally extending forward from the head of the suction piston 3 with a metering jet 9 strikes upon the head of the suction piston due to inertia thereof immediately after being ejected from the main nozzle 5, and only a part of the fuel is atomized and flows down into a mixing chamber 10.
Then, the fuel which struck upon the head of the suction piston 3 and attached thereon flows down along the surface of the head and collects together on the end surface 11 at the downstream side to grow up to a large fuel droplet 12. When the force of gravity acting on the fuel droplet exceeds the surface tension thereof, the fuel droplet drops upon the trottle valve 2 as illustrated in the drawing, thus resulting in an increased air/fuel ratio at each drop.
Furthermore, since the fuel droplets 12 grow periodically in time and hence drip periodically, it exerts a great influence, particularly when an amount of supplied fuel is small, for example, during idling. As shown in FIG. 2, where the abscissa represents time T and the ordinate represents and air/fuel ratio A/F, a disadvantage has been encountered such that the air/fuel ratio becomes rich in the form of a spike synchronous with the dropping of the fuel droplet, thus leading to drawbacks such that the rpm of the engine varies every time the fuel droplet 12 drops, making the idling unstable.